This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The goal of this study is to understand the role of the HIV-1 protein Vpu in HIV pathogenesis. Our previous studies demonstrated that Vpu modulates expression of the adhesion molecule VCAM-1 on endothelial cells (EC) via induction of the cytokine receptor CD40. Modulation of the endothelium in this manner may contribute to the characteristic extranodal presentation of AIDS-associated B cell lymphomas. Ongoing studies have shown that Vpu's effect on CD40 is likely the downstream consequence of Vpu modulation of another protein or pathway. We thus undertook an MS-based proteomics approach, stable isotope labeling with amino acids in cell culture (SILAC), to identify novel cellular proteins affected by Vpu. We compared the protein profiles of Hela-CD4 cells expressing wildtype Vpu to the same cell line expressing a mutant of Vpu that does not interact with beta-TrCP and thus cannot interact with the Ubiquitin-proteosome pathway for stabilization or degradation of Vpu targets. We identified 7 proteins that were up- or downregulated at least 1.5-fold by wildtype Vpu. One of these proteins, BST-2, was recently identified as a host antiviral factor whose function is to restrict the release of HIV virions from infected cells. The protein was thus renamed tetherin. Our novel finding that HIV Vpu could downregulate surface expression of BST-2/tetherin reveals a mechanism for HIV to counteract the host restriction. Based on the potential significance of these findings, we are focusing our current efforts on determining the mechanism and significance of Vpu-mediated BST-2 downregulation.